4.2.4.1 Independent tanks
are self-supporting; they do not form part of the ship's hull and
are not essential to the hull strength. There are three categories
of independent tanks referred to in 4.2.4.2, 4.2.4.3 and 4.2.4.4.
4.2.4.2 Type A independent
tanks are tanks which are designed primarily using recognized standards
of classical ship-structural analysis procedures. Where such tanks
are primarily constructed of plane surfaces (gravity tanks), the design
vapour pressure P
o should be less than 0.7
bar.
4.2.4.3 Type B independent
tanks are tanks which are designed using model tests, refined analytical
tools and analysis methods to determine stress levels, fatigue life
and crack propagation characteristics. Where such tanks are primarily
constructed of plane surfaces (gravity tanks) the design vapour pressure P
o should be less than 0.7 bar.
4.2.4.4 Type C independent
tanks (also referred to as pressure vessels) are tanks meeting pressure
vessel criteria and having a design vapour pressure not less than:
where:
|
A
|
= |
|
with
|
σm
|
= |
design
primary membrane stress |
|
ΔσA
|
= |
allowable dynamic membrane stress (double amplitude at probability
level Q = 10-8)
|
|
|
= |
55 N/mm2 for ferritic-perlitic,
martensitic and austenitic steel
|
|
|
= |
25 N/mm2 for aluminium
alloy (5083-0)
|
|
C
|
= |
a
characteristic tank dimension to be taken as the greatest of the following: |
with
|
h
|
= |
height
of tank (dimension in ship's vertical direction) (m) |
|
b
|
= |
width
of tank (dimension in ship's transverse direction) (m) |
|
l
|
= |
length
of tank (dimension in ship's longitudinal direction) (m) |
|
ρr
|
= |
the
relative density of the cargo (ρr = 1 for fresh water)
at the design temperature.
|
However, the Administration may allocate a tank complying
with the criterion of this subparagraph to type A or type B, dependent
on the configuration of the tank and the arrangement of its supports
and attachments.